I am sitting in a make-up chair staring at my new military haircut. The hairdresser is on cheerful auto-pilot; I am just one of a hundred heads she will buzz this month. I look down the line and see other 20-something men wearing the same nervous grin I had worn only moments before. As the hum of the trimmer slowly dies, it seems we're ready for war.

My few days are spent 55km south-west of Melbourne among the dramatic granite peaks of the You Yangs. As I step out of my trailer and on to the set of the most expensive television mini-series ever made, it's clear that the detail and scale of this production are awesome – the plastic palm tree and the plastic flowering vine wrapping round it even fool a few local birds, who dart in and out of the artificial leaves.

My scenes are shot in a wooden shelter that stands as the psychiatric ward of a hospital on Banika Island. I play Captain Midnight in episode four – a second world war pilot suffering from shell shock and keeping people up at night (hence his nickname). I got the role after months of auditions for another bigger role, and with my American accent in my pocket, my metaphorical guns blazing and my fight choreography down, Captain Midnight comes noisily to life.

Despite the haircut, I don't actually get to fight. But I do get to hear the fighting: shooting with Blue Unit in the make-shift hospital, production would have to occasionally stop, or one of the assistant directors would frantically communicate with the Red Unit – simultaneously shooting a fierce battle scene some distance away – as they rolled with their massive explosives.

There was nothing plastic about those sounds. With each boom the image of soldiers facing relentless enemy fire became frightening and all-consuming – I was thankful to be in hospital and not on the frontline. Actors who were say that the dirt, the mud, the wet, the fear inside the trenches, the nerves before the battle – all of it felt incredibly real. But while spending time cold and wet, or filthy and sweating, was uncomfortable, the actors actually bonded over the conditions. In any case, complaining about the conditions seemed redundant – unimaginable even – given the historical significance of the exercise.

You can't escape that. But back in my trailer at the end of a long day, I also can't help but think how cool it is to be in an HBO mini-series, produced by Steven Spielberg and Tom Hanks; to be in an episode directed by the guy who wrote Speed (whose single direction to me is to "shake my hand more" in the nervous tick that Captain Midnight has developed since arriving at the hospital). If this guy can write dialogue that Keanu Reeves can pull off then I am in good hands. In fact, I'm in actor heaven.